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        <h2 class="post-title" itemprop="name headline">c++11的模板类型判断std::is_same和std::decay

          
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        <p>问题提出：有一个模板函数，函数在处理int型和double型时需要进行特殊的处理，那么怎么在编译期知道传入的参数的数据类型是int型还是double型呢？<br>如：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;iostream&gt;</span></span></span><br><span class="line"><span class="keyword">template</span> &lt;<span class="keyword">typename</span> TYPE&gt;</span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">typeCheck</span><span class="params">(TYPE data)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="comment">//do something check data type</span></span><br><span class="line">	<span class="comment">//std::cout&lt;&lt; out put the type</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>这里就需要用到C++11的type_traits头文件了，type_traits头文件定义了很多类型检查相关的方法，上面的例子具体用到了其中两个结构：</p>
<h2 id="std-is-same-判断类型是否一致"><a href="#std-is-same-判断类型是否一致" class="headerlink" title="std::is_same 判断类型是否一致"></a>std::is_same 判断类型是否一致</h2><p>位于头文件<code>&lt;type_traits&gt;</code>中<br>这个结构体作用很简单，就是两个一样的类型会返回true</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">bool</span> isInt = <span class="built_in">std</span>::is_same&lt;<span class="keyword">int</span>, <span class="keyword">int</span>&gt;::value; <span class="comment">//为true</span></span><br></pre></td></tr></table></figure>

<p>下面是官方的例子：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;iostream&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;type_traits&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;cstdint&gt;</span></span></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">print_separator</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="string">"-----\n"</span>;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="built_in">std</span>::boolalpha;</span><br><span class="line"></span><br><span class="line">    <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="built_in">std</span>::is_same&lt;<span class="keyword">int</span>, <span class="keyword">int32_t</span>&gt;::value &lt;&lt; <span class="string">'\n'</span>;   <span class="comment">// true</span></span><br><span class="line">    <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="built_in">std</span>::is_same&lt;<span class="keyword">int</span>, <span class="keyword">int64_t</span>&gt;::value &lt;&lt; <span class="string">'\n'</span>;   <span class="comment">// false</span></span><br><span class="line">    <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="built_in">std</span>::is_same&lt;<span class="keyword">float</span>, <span class="keyword">int32_t</span>&gt;::value &lt;&lt; <span class="string">'\n'</span>; <span class="comment">// false</span></span><br><span class="line"></span><br><span class="line">    print_separator();</span><br><span class="line"></span><br><span class="line">    <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="built_in">std</span>::is_same&lt;<span class="keyword">int</span>, <span class="keyword">int</span>&gt;::value &lt;&lt; <span class="string">"\n"</span>;          <span class="comment">// true</span></span><br><span class="line">    <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="built_in">std</span>::is_same&lt;<span class="keyword">int</span>, <span class="keyword">unsigned</span> <span class="keyword">int</span>&gt;::value &lt;&lt; <span class="string">"\n"</span>; <span class="comment">// false</span></span><br><span class="line">    <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="built_in">std</span>::is_same&lt;<span class="keyword">int</span>, <span class="keyword">signed</span> <span class="keyword">int</span>&gt;::value &lt;&lt; <span class="string">"\n"</span>;   <span class="comment">// true</span></span><br><span class="line"></span><br><span class="line">    print_separator();</span><br><span class="line"></span><br><span class="line">    <span class="comment">// unlike other types 'char' is not 'unsigned' and not 'signed'</span></span><br><span class="line">    <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="built_in">std</span>::is_same&lt;<span class="keyword">char</span>, <span class="keyword">char</span>&gt;::value &lt;&lt; <span class="string">"\n"</span>;          <span class="comment">// true</span></span><br><span class="line">    <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="built_in">std</span>::is_same&lt;<span class="keyword">char</span>, <span class="keyword">unsigned</span> <span class="keyword">char</span>&gt;::value &lt;&lt; <span class="string">"\n"</span>; <span class="comment">// false</span></span><br><span class="line">    <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="built_in">std</span>::is_same&lt;<span class="keyword">char</span>, <span class="keyword">signed</span> <span class="keyword">char</span>&gt;::value &lt;&lt; <span class="string">"\n"</span>;   <span class="comment">// false</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>通过std::is_same即可判断两个类型是否一样，特别在模板里面，在不清楚模板的参数时，此功能可以对一些特定的参数类型进行特殊的处理。</p>
<a id="more"></a>

<blockquote>
<p>这里说个题外话，大家是否通过std::is_same发现，char既不是unsigned char也不是signed char，char就是char，这和int是signed int的缩写是不一样的，char的表达范围可能等同于signed char，也可能等同于unsigned char，取决于编译器，一般是等同于signed char，但这个仅仅是范围等同，就像32位上int和long范围是一样的，但不是同一个类型。</p>
<p>因为用途不同，char用于表达字符，理论上不应该关心其正负的实现，而signed char 和 unsigned char 用于表达数值，或可移植的char。</p>
</blockquote>
<p>回到正文，std::is_same可以判断两种类似是否一样，那么用在模板里就是利器了，本位一开始提到的那个问题就可以这样写：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;iostream&gt;</span></span></span><br><span class="line"><span class="keyword">template</span>&lt;<span class="keyword">typename</span> TYPE&gt;</span><br><span class="line">typeCheck(TYPE data)</span><br><span class="line">&#123;</span><br><span class="line">    <span class="keyword">if</span>(<span class="built_in">std</span>::is_same&lt;TYPE,<span class="keyword">int</span>&gt;::value)</span><br><span class="line">    &#123;</span><br><span class="line">        <span class="built_in">std</span>::<span class="built_in">cout</span>&lt;&lt;<span class="string">"int type"</span>;</span><br><span class="line">        <span class="comment">//do something int </span></span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">else</span></span><br><span class="line">    &#123;</span><br><span class="line">        <span class="comment">//.........</span></span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>看似很美好，再看一个示例：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// is_same example</span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;iostream&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;type_traits&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;cstdint&gt;</span></span></span><br><span class="line"></span><br><span class="line"><span class="keyword">typedef</span> <span class="keyword">int</span> integer_type;</span><br><span class="line"><span class="class"><span class="keyword">struct</span> <span class="title">A</span> &#123;</span> <span class="keyword">int</span> x,y; &#125;;</span><br><span class="line"><span class="class"><span class="keyword">struct</span> <span class="title">B</span> &#123;</span> <span class="keyword">int</span> x,y; &#125;;</span><br><span class="line"><span class="keyword">typedef</span> A C;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">()</span> </span>&#123;</span><br><span class="line">      <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="built_in">std</span>::boolalpha;</span><br><span class="line">      <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="string">"is_same:"</span> &lt;&lt; <span class="built_in">std</span>::<span class="built_in">endl</span>;</span><br><span class="line">      <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="string">"int, const int: "</span> &lt;&lt; <span class="built_in">std</span>::is_same&lt;<span class="keyword">int</span>, <span class="keyword">const</span> <span class="keyword">int</span>&gt;::value &lt;&lt; <span class="built_in">std</span>::<span class="built_in">endl</span>;<span class="comment">//false</span></span><br><span class="line">      <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="string">"int, int&amp;: "</span> &lt;&lt; <span class="built_in">std</span>::is_same&lt;<span class="keyword">int</span>, <span class="keyword">int</span>&amp;&gt;::value &lt;&lt; <span class="built_in">std</span>::<span class="built_in">endl</span>;<span class="comment">//false</span></span><br><span class="line">      <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="string">"int, const int&amp;: "</span> &lt;&lt; <span class="built_in">std</span>::is_same&lt;<span class="keyword">int</span>, <span class="keyword">const</span> <span class="keyword">int</span>&amp;&gt;::value &lt;&lt; <span class="built_in">std</span>::<span class="built_in">endl</span>;<span class="comment">//false</span></span><br><span class="line">      <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="string">"int, integer_type: "</span> &lt;&lt; <span class="built_in">std</span>::is_same&lt;<span class="keyword">int</span>, integer_type&gt;::value &lt;&lt; <span class="built_in">std</span>::<span class="built_in">endl</span>;<span class="comment">//true</span></span><br><span class="line">      <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="string">"A, B: "</span> &lt;&lt; <span class="built_in">std</span>::is_same&lt;A,B&gt;::value &lt;&lt; <span class="built_in">std</span>::<span class="built_in">endl</span>;<span class="comment">//false</span></span><br><span class="line">      <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="string">"A, C: "</span> &lt;&lt; <span class="built_in">std</span>::is_same&lt;A,C&gt;::value &lt;&lt; <span class="built_in">std</span>::<span class="built_in">endl</span>;<span class="comment">//true</span></span><br><span class="line">      <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="string">"signed char, std::int8_t: "</span> &lt;&lt; <span class="built_in">std</span>::is_same&lt;<span class="keyword">signed</span> <span class="keyword">char</span>,<span class="built_in">std</span>::<span class="keyword">int8_t</span>&gt;::value &lt;&lt; <span class="built_in">std</span>::<span class="built_in">endl</span>;<span class="comment">//true</span></span><br><span class="line">      <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>输出：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">is_same:</span><br><span class="line"><span class="keyword">int</span>, <span class="keyword">const</span> <span class="keyword">int</span>: <span class="literal">false</span></span><br><span class="line"><span class="keyword">int</span>, <span class="keyword">int</span>&amp;: <span class="literal">false</span></span><br><span class="line"><span class="keyword">int</span>, <span class="keyword">const</span> <span class="keyword">int</span>&amp;: <span class="literal">false</span></span><br><span class="line"><span class="keyword">int</span>, integer_type: <span class="literal">true</span></span><br><span class="line">A, B: <span class="literal">false</span></span><br><span class="line">A, C: <span class="literal">true</span></span><br><span class="line"><span class="keyword">signed</span> <span class="keyword">char</span>, <span class="built_in">std</span>::<span class="keyword">int8_t</span>: <span class="literal">true</span></span><br></pre></td></tr></table></figure>

<p>可以发现std::is_same的判断是很严格的,再看下面的一个例子：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;stdlib.h&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;iostream&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;type_traits&gt;</span></span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span>&lt;<span class="keyword">typename</span> TYPE&gt;</span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">typeCheck</span><span class="params">(TYPE data)</span></span>;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="keyword">int</span> a = <span class="number">1</span>;</span><br><span class="line">    <span class="keyword">const</span> <span class="keyword">int</span>&amp; b = a;</span><br><span class="line">    <span class="keyword">int</span>&amp; c = a;</span><br><span class="line">    <span class="keyword">int</span> d[<span class="number">12</span>];</span><br><span class="line">    <span class="keyword">const</span> <span class="keyword">int</span>&amp; e = d[<span class="number">7</span>];</span><br><span class="line">    typeCheck(a);<span class="comment">//int type</span></span><br><span class="line">    typeCheck(b);<span class="comment">//int type</span></span><br><span class="line">    typeCheck(c);<span class="comment">//int type</span></span><br><span class="line">    typeCheck(d[<span class="number">7</span>]);<span class="comment">//int type</span></span><br><span class="line">    typeCheck(e);<span class="comment">//int type</span></span><br><span class="line">    typeCheck(<span class="number">8</span>);<span class="comment">//int type</span></span><br><span class="line">    <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span>&lt;<span class="keyword">typename</span> TYPE&gt;</span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">typeCheck</span><span class="params">(TYPE data)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="keyword">if</span>(<span class="built_in">std</span>::is_same&lt;TYPE,<span class="keyword">int</span>&gt;::value)</span><br><span class="line">    &#123;</span><br><span class="line">        <span class="built_in">std</span>::<span class="built_in">cout</span>&lt;&lt;<span class="string">"int type"</span>&lt;&lt;<span class="built_in">std</span>::<span class="built_in">endl</span>;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">else</span> <span class="keyword">if</span>(<span class="built_in">std</span>::is_same&lt;TYPE,<span class="built_in">std</span>::<span class="built_in">string</span>&gt;::value)</span><br><span class="line">    &#123;</span><br><span class="line">        <span class="built_in">std</span>::<span class="built_in">cout</span>&lt;&lt;<span class="string">"string type"</span>&lt;&lt;<span class="built_in">std</span>::<span class="built_in">endl</span>;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">else</span></span><br><span class="line">    &#123;</span><br><span class="line">        <span class="built_in">std</span>::<span class="built_in">cout</span>&lt;&lt;<span class="string">"other type"</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>输出：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> type</span><br><span class="line"><span class="keyword">int</span> type</span><br><span class="line"><span class="keyword">int</span> type</span><br><span class="line"><span class="keyword">int</span> type</span><br><span class="line"><span class="keyword">int</span> type</span><br><span class="line"><span class="keyword">int</span> type</span><br></pre></td></tr></table></figure>

<p>测试后发现，虽然变量b,c, e使用的是引用，std::is_same那么严格为什么是int_type呢? 因为在写模板函数时，经常会强制指定const引用进行传参，以免进行数据拷贝，这时候is_same就做出了相等的判断.</p>
<blockquote>
<p>如果我们显示的指定模板参数类型时情况有不一样了：</p>
</blockquote>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;stdlib.h&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;iostream&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;type_traits&gt;</span></span></span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span>&lt;<span class="keyword">typename</span> TYPE&gt;</span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">typeCheck</span><span class="params">(TYPE data)</span></span>;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="keyword">int</span> a = <span class="number">1</span>;</span><br><span class="line">    <span class="keyword">const</span> <span class="keyword">int</span>&amp; b = a;</span><br><span class="line">    <span class="keyword">int</span>&amp; c = a;</span><br><span class="line">    <span class="keyword">int</span> d[<span class="number">12</span>];</span><br><span class="line"></span><br><span class="line">    typeCheck&lt;<span class="keyword">int</span>&gt;(a);        <span class="comment">//int type</span></span><br><span class="line">    typeCheck&lt;<span class="keyword">const</span> <span class="keyword">int</span>&amp;&gt;(b);<span class="comment">//other type</span></span><br><span class="line">    typeCheck&lt;<span class="keyword">int</span> &amp;&gt;(c);        <span class="comment">//other type</span></span><br><span class="line">    typeCheck&lt;<span class="keyword">const</span> <span class="keyword">int</span>&amp;&gt;(d[<span class="number">7</span>]);<span class="comment">//other type</span></span><br><span class="line">    typeCheck(<span class="number">8</span>);                <span class="comment">//int type</span></span><br><span class="line">    <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span>&lt;<span class="keyword">typename</span> TYPE&gt;</span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">typeCheck</span><span class="params">(TYPE data)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="keyword">if</span>(<span class="built_in">std</span>::is_same&lt;TYPE,<span class="keyword">int</span>&gt;::value)</span><br><span class="line">    &#123;</span><br><span class="line">        <span class="built_in">std</span>::<span class="built_in">cout</span>&lt;&lt;<span class="string">"int type"</span>&lt;&lt;<span class="built_in">std</span>::<span class="built_in">endl</span>;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">else</span> <span class="keyword">if</span>(<span class="built_in">std</span>::is_same&lt;TYPE,<span class="built_in">std</span>::<span class="built_in">string</span>&gt;::value)</span><br><span class="line">    &#123;</span><br><span class="line">        <span class="built_in">std</span>::<span class="built_in">cout</span>&lt;&lt;<span class="string">"string type"</span>&lt;&lt;<span class="built_in">std</span>::<span class="built_in">endl</span>;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">else</span></span><br><span class="line">    &#123;</span><br><span class="line">        <span class="built_in">std</span>::<span class="built_in">cout</span>&lt;&lt;<span class="string">"other type"</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>输出：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> type</span><br><span class="line">other type</span><br><span class="line">other type</span><br><span class="line">other type</span><br><span class="line"><span class="keyword">int</span> type</span><br></pre></td></tr></table></figure>

<p>瞬间结果就不一样了，这很好了解，从上面可知道，std::is_same对int\ const int\ int &amp;\ const int&amp; 等都是区别对待的.</p>
<blockquote>
<p>但是有时候其实我们还是希望TYPE和const TYPE&amp; 是能认为是一样的，这时就需要std::decay进行退化处理</p>
</blockquote>
<h2 id="std-decay-退化类型的修饰"><a href="#std-decay-退化类型的修饰" class="headerlink" title="std::decay 退化类型的修饰"></a>std::decay 退化类型的修饰</h2><p>std::decay就是对一个类型进行退化处理，他的实现如下:</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">template</span>&lt; <span class="class"><span class="keyword">class</span> <span class="title">T</span> &gt;</span></span><br><span class="line"><span class="class"><span class="title">struct</span> <span class="title">decay</span> &#123;</span></span><br><span class="line"><span class="keyword">private</span>:</span><br><span class="line">    <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="built_in">std</span>::remove_reference&lt;T&gt;::type U;</span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line">    <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="built_in">std</span>::conditional&lt; </span><br><span class="line">        <span class="built_in">std</span>::is_array&lt;U&gt;::value,</span><br><span class="line">        <span class="keyword">typename</span> <span class="built_in">std</span>::remove_extent&lt;U&gt;::type*,</span><br><span class="line">        <span class="keyword">typename</span> <span class="built_in">std</span>::conditional&lt; </span><br><span class="line">            <span class="built_in">std</span>::is_function&lt;U&gt;::value,</span><br><span class="line">            <span class="keyword">typename</span> <span class="built_in">std</span>::add_pointer&lt;U&gt;::type,</span><br><span class="line">            <span class="keyword">typename</span> <span class="built_in">std</span>::remove_cv&lt;U&gt;::type</span><br><span class="line">        &gt;::type</span><br><span class="line">    &gt;::type type;</span><br><span class="line">&#125;;</span><br></pre></td></tr></table></figure>

<p>看着比较抽象，其实就是把各种引用啊什么的修饰去掉，把cosnt int&amp;退化为int，这样就能通过std::is_same正确识别出加了引用的类型了<br>上面的例子改为：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;iostream&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;type_traits&gt;</span></span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span>&lt;<span class="keyword">typename</span> TYPE&gt;</span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">typeCheck</span><span class="params">(TYPE data)</span></span>;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="keyword">int</span> a = <span class="number">1</span>;</span><br><span class="line">    <span class="keyword">const</span> <span class="keyword">int</span>&amp; b = a;</span><br><span class="line">    <span class="keyword">int</span>&amp; c = a;</span><br><span class="line">    <span class="keyword">int</span> d[<span class="number">12</span>];</span><br><span class="line"></span><br><span class="line">    typeCheck&lt;<span class="keyword">int</span>&gt;(a);<span class="comment">//int type</span></span><br><span class="line">    typeCheck&lt;<span class="keyword">const</span> <span class="keyword">int</span>&amp;&gt;(b);<span class="comment">//int type</span></span><br><span class="line">    typeCheck&lt;<span class="keyword">int</span> &amp;&gt;(c);<span class="comment">//int type</span></span><br><span class="line">    typeCheck&lt;<span class="keyword">const</span> <span class="keyword">int</span>&amp;&gt;(d[<span class="number">7</span>]);<span class="comment">//int type</span></span><br><span class="line">    typeCheck(<span class="number">8</span>);<span class="comment">//int type</span></span><br><span class="line">    <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span>&lt;<span class="keyword">typename</span> TYPE&gt;</span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">typeCheck</span><span class="params">(TYPE data)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="keyword">if</span>(<span class="built_in">std</span>::is_same&lt;<span class="keyword">typename</span> <span class="built_in">std</span>::decay&lt;TYPE&gt;::type,<span class="keyword">int</span>&gt;::value)<span class="comment">//c++11</span></span><br><span class="line">    <span class="comment">//if(std::is_same&lt;std::decay_t&lt;TYPE&gt;, int&gt;::value)//c++14</span></span><br><span class="line">    &#123;</span><br><span class="line">        <span class="built_in">std</span>::<span class="built_in">cout</span>&lt;&lt;<span class="string">"int type"</span>&lt;&lt;<span class="built_in">std</span>::<span class="built_in">endl</span>;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">else</span></span><br><span class="line">    &#123;</span><br><span class="line">        <span class="built_in">std</span>::<span class="built_in">cout</span>&lt;&lt;<span class="string">"other type"</span>&lt;&lt;<span class="built_in">std</span>::<span class="built_in">endl</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>在cppref有个更加详细的例子：</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;iostream&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">include</span> <span class="meta-string">&lt;type_traits&gt;</span></span></span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span> &lt;<span class="keyword">typename</span> T, <span class="keyword">typename</span> U&gt;</span><br><span class="line"><span class="class"><span class="keyword">struct</span> <span class="title">decay_equiv</span> :</span> </span><br><span class="line">    <span class="built_in">std</span>::is_same&lt;<span class="keyword">typename</span> <span class="built_in">std</span>::decay&lt;T&gt;::type, U&gt;::type </span><br><span class="line">&#123;&#125;;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="built_in">std</span>::<span class="built_in">cout</span> &lt;&lt; <span class="built_in">std</span>::boolalpha</span><br><span class="line">              &lt;&lt; decay_equiv&lt;<span class="keyword">int</span>, <span class="keyword">int</span>&gt;::value &lt;&lt; <span class="string">'\n'</span></span><br><span class="line">              &lt;&lt; decay_equiv&lt;<span class="keyword">int</span>&amp;, <span class="keyword">int</span>&gt;::value &lt;&lt; <span class="string">'\n'</span></span><br><span class="line">              &lt;&lt; decay_equiv&lt;<span class="keyword">int</span>&amp;&amp;, <span class="keyword">int</span>&gt;::value &lt;&lt; <span class="string">'\n'</span></span><br><span class="line">              &lt;&lt; decay_equiv&lt;<span class="keyword">const</span> <span class="keyword">int</span>&amp;, <span class="keyword">int</span>&gt;::value &lt;&lt; <span class="string">'\n'</span></span><br><span class="line">              &lt;&lt; decay_equiv&lt;<span class="keyword">int</span>[<span class="number">2</span>], <span class="keyword">int</span>*&gt;::value &lt;&lt; <span class="string">'\n'</span></span><br><span class="line">              &lt;&lt; decay_equiv&lt;<span class="keyword">int</span>(<span class="keyword">int</span>), <span class="keyword">int</span>(*)(<span class="keyword">int</span>)&gt;::value &lt;&lt; <span class="string">'\n'</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>输出:</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="literal">true</span></span><br><span class="line"><span class="literal">true</span></span><br><span class="line"><span class="literal">true</span></span><br><span class="line"><span class="literal">true</span></span><br><span class="line"><span class="literal">true</span></span><br><span class="line"><span class="literal">true</span></span><br></pre></td></tr></table></figure>

<h2 id="总结："><a href="#总结：" class="headerlink" title="总结："></a>总结：</h2><ul>
<li><p>在模板里可以通过std::is_same判断模板的类型，从而实现对不同类型的区别对待</p>
</li>
<li><p>在堆类型要求不是非常严格的情况下，可以使用std::decay把类型退化为基本形态，结合std::is_same用，可以判断出更多的情况</p>
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